Spell Queller

If the card has X generic mana in its mana cost, the player must choose 0 as the value of X when casting it without paying its mana cost.

If a player casts a card “without paying its mana cost,” they can’t choose to cast it for any alternative costs, such as emerge costs. The player can, however, pay additional costs, such as escalate costs. If the card has any mandatory additional costs, those must be paid to cast the card.

If Spell Queller leaves the battlefield before its “enters” ability resolves, its leaves-the-battlefield triggered ability triggers, resolves, and does nothing. Then its first triggered ability resolves and exiles the spell forever.